This invention relates generally to cable control systems, and more particularly, to such a system having a fail-safe mechanism to isolate a controlled member from the release of stored energy when a cable breaks or becomes disconnected.
Cables are often used to control flight control surfaces such as elevators, rudders or ailerons, or to control valves or other mechanisms in an airplane. Control equipment in other vehicles or various other types of machinery may also employ closed loop cable control systems. In a typical arrangement, each of two cables extend between an input pulley and an output pulley. Ends of the cables are connected on opposite sides of each pulley. The input pulley is connected to a lever which is connected by suitable linkage to an input mechanism, such as the steering column of an aircraft. Movement of the steering column results in rotation of the input pulley about its axis of rotation. The input pulley rotation is transmitted by the cables to the output pulley, causing a corresponding rotation. The rotary motion can be used directly to actuate a controlled member, or can be transmitted by a linkage connected to a lever on the output pulley.
In order for cable control systems to function properly, the cables must not be allowed to go slack. In the situation where these systems are used in aircraft, they are subjected to significant loads, load variations and thermal stresses in operation. As a result, there is a tendency for elongation or contraction of the cables as well as deformation of the structure supporting the cables. These changes can cause a loss of control function. To prevent the cables from going slack, the cables are rigged in the cable control system under a substantial load producing high tension in the cables. The tension in the cables allows the cable control system to remain operational despite elongation and contraction of the cables.
It is known that cables may break or otherwise become disconnected from the pulleys. When such failures occur substantially instantaneously, the energy which has been stored up as tension in the cables is violently released. The broken cable moves with great velocity and the unbroken cable recoils as its tension is relieved. A substantial force is imparted to the output pulley by the release of stored energy which can cause the pulley to pivot rapidly about its axis of rotation in an uncontrolled manner. Thus, the controlled member may be violently moved to some position.
There are cable control systems which are designed to permit retention of control when one of the cables breaks. Some use spring mechanisms to convert a dual cable control into a single cable control system. In the single cable control system, the spring urges the pulley to rotate in one direction and the remaining cable operates against the spring to produce rotation in the other direction. Other cable control systems disconnect the input pulley so that it can receive no input to transmit to the output pulley. In that event, there is a redundant control which takes over operation. However, these control systems do not isolate the output pulley and controlled member from forces created by the sudden release of the cable tension.